Which is not a good nucleophile, phenoxide ion or acetate ion?

Question

Sarah Angotti · Accepted Answer

To make donation easier, the superior nucleophile should, in general, have a higher electron density concentrated onto a single atom.

Whereas the phenoxide ion is simply the aromatic phenol anion, #"C"_6"H"_5-"O"^(-)#, with a "well-defined" HOMO (located on the negatively-charged oxygen), the acetate ion has delocalized electron density across its two oxygens. That is, it is shared between the two oxygens, and its HOMO is less "well-defined".

It is more difficult to donate electrons onto an electrophile's LUMO during the reaction mechanism because of the acetate ion's more dispersed electron density (it's like saying "protip:... " and not having anything to show for it!).

Understanding this could be achieved by realizing that moving electron density from one location (the nucleophilic atom) to another (the electrophilic atom) becomes more difficult because the electron density must first be gathered in one location.

As a result, the phenoxide ion ought to be the more favorable nucleophile—not necessarily GOOD, but still preferable to the acetate ion.

Nucleophilicity is always relative!

Zoe Alexander · Answer

undefined Acetate ion.